CMFActivity.Activity.SQLBase: Avoid reference loops.

Unsure whether this makes a difference, but let's be on the safe side.

product/CMFActivity/Activity/SQLBase.py

@@ -664,6 +664,7 @@ CREATE TABLE %s (
             % [(m.uid, m.object_path, m.method_id) for m in message_list])
           for m in message_list:
             m.setExecutionState(MESSAGE_NOT_EXECUTED, exc_info, log=False)
+        del exc_info
         self._abort()
         exc_info = message_list[0].exc_info
         if exc_info:
@@ -671,7 +672,6 @@ CREATE TABLE %s (
             # Register it again.
             with activity_runtime_environment:
               cancel = message.on_error_callback(*exc_info)
-            del exc_info, message.exc_info
             transaction.commit()
             if cancel:
               message.setExecutionState(MESSAGE_EXECUTED)
@@ -679,6 +679,7 @@ CREATE TABLE %s (
             self._log(WARNING, 'Exception raised when processing error callbacks')
             message.setExecutionState(MESSAGE_NOT_EXECUTED)
             self._abort()
+          del exc_info, message.exc_info
       self.finalizeMessageExecution(activity_tool, message_list,
                                     uid_to_duplicate_uid_list_dict)
     transaction.commit()